It seems pretty clear that the West Lothian question will come to a head if Gordon Brown becomes British PM.
There's a great discussion over at Albion's Seedlings on this; this comment summarizes the issue quite well (emphasis mine):
Brown cannot be UK Prime Minister without sparking a constitutional crisis. His mandate is from his constituents in Scotland who elect him on matters that are reserved by the UK legislature, they do not elect him on matters devolved to the Scottish Parliament (health, education, transport, culture, sport etc).
On all the policy areas that have been devolved to Scotland he has no mandate to govern England because he is unaccountable to the people that he seeks to govern.
Labour has a majority of 67 which is dependent on their Scottish and Welsh MPs. The English will not stand for a democratically unaccountable Prime Minister using Scottish and Welsh MPs to legislate for England whilst English MPs have no say in the concommitant legislation in outwith England…
If he wants to lead the UK then Brown has to become an English nationalist and promote English self-governance. Only by removing from his remit those legislative areas that he is unaccountable to the electorate on, and placing them in the hands of an English government, can he avoid a total breakdown in government.
We don't run into these sorts of issues because we have a very clear federal structure. I think the British Constitution will be dragged, perhaps kicking and screaming, into the 18th century.